Limiter circuit for frequency modulation receivers



Dec. 11, 1945.

c. E. ATKINS 2,390,502

LIMITER CIRCUIT FOR FREQUENCY MODULATION RECEIVERS Filed Jan. 7 1942 2 Sheets-Sheet 1 INVENTOR T/Z' (/AFL E. Arm/v5 ww ma ATTORNEYS Dec. 11, 1945.

C. E. ATKINS Filed Jan. 7, 1942 LIMITER CIRCUIT FOR FREQUENCY MODULATION RECEIVERS .8 Sheets-Sheet 2 2 n II INVENTOR waifgqm u m ATTORNEYS M Dec. 11, 1945 LIMITER CIRCUIT FOR FREQUENCY MODULATION RECEIVERS Carl E. Atkins, Oak Park, 111., assignor to Tung- Sol Lamp Works, Inc., Newark, N. 1., a corpo'ration of Delaware Application January-1, 1942, Serial No. 425,843

, 11 Claims.

The present invention relates to radio reception and more particularly to the reception of frequency modulated waves and comprises a novel limiter circuit for incorporation into radio receivers of this general typ In frequency modulated radio reception, as distinguished from amplitude modulated radio reception, the receivers should respond only to the rate of chang of angle of the incoming wave, that is to the relative frequency variation of the wave and should not be responsive to excessive amplitude variations 01 the wave. Accordingly, receivers of this type include in one form or another a device for suppressing amplitude variations above a predetermined value, which excess variations are caused by static or noise impulses. Such devices are known as limiters and consis principally oi one or more amplifier stages operated under conditions of poor eiiiciency so that the gain per stage is necessarily low and the stage will overload when a, signal of any magnitude is applied to it. Usually these stages include resistor-condenser net works in the amplifier tube grid return. Various disadvantages are inherent in the above type of limiter. It is not able to follow rapid impulses, because oi the time consumed in discharge of the condenser through the resistances, and hence is not effective to suppress short duration impulse noises. Also, there is a. tend- 'ency, after becoming severely overloaded, for

which non-linear impedances are utilized for impressing degenerative potentials on the control grid of the amplifier when the output potentials exceed the limiting value;

Fig. 2 is a diagrammatic circuit drawing of a V circuits similar to Fig. 1 but wherein the feedback operates inductively upon the input circuit of the amplifier.

In the different drawings like elements are indicated by the same reference numerals or letters.

The limiter circuit of Fig. 1 includes the amplifier tube 2, which may be of any well-known type, preferably one having a screen grid, as for example, the GSD'IGT. A suitable input circuit for the tube 2 isindicated as the conventional tuned circuit having the inductance L bridged by the condenser C, the circuit being grounded at one end and connected to the control grid 4 at voltage through the load Z, here shown as a resistance. Output connection for the plate circuit is indicated at I 0.

A pair of diodes, specifically shown in Fig. 1

.as elements within a single tube of the type inherently rapid in response and hence will suppress noise impulses of short, as well as long, duration.

In the limiter of the present invention, the resistance-condenser network is omitted and a teed-back between output and input utilizing non-linear impedances is provided for insuring degeneration of the amplifier tube when the amplitude variations exceed a predetermined value. In accordance withthe invention the non-linear impedances are used to pass current through the 7 feed back when the output potentials exceed a known as a 6H6, have an anode i2 of one diode and a cathode I4 of the other diode tied together and connected by a lead I5 to thecontrol grid 4 of the tube 2. The anode ii of the last men tioned diode is connected through a radio frequency choke l8 with the negative terminal of a battery 20, the positive terminal of which is grounded, and the cathode 22 of the first; mentioned diode is similarlyv connected through a radio frequency choke 24 to the positive terminal of a battery 26, the negative terminal oi which is grounded. Cathode 22 and anode ii are connected to the plate 8 of tube 2 through blocking. condenser C2 and C3, respectively.

The operation of the circuit of Fig. 1 willnow be described. Radio frequency signal voltage appearing across the input or tank circuit LC is applied to the control grid 4 of the-amplifier tube 2, whereupon current flows in the plate circuit including the resistance Z, the voltage across which becomes available for application to a succeeding amplifier tube. The voltage across Z many times greater than the input voltage across 14C. and as th input voltage increases,

the output voltage will correspondingly increase until the potential applied to the diodes is sufflcient-to override the polarizing potentials applied thereto by the batteries 20 and 26. When this occurs, the diodes permit passage of current therethrough and a part of the output current is thus fed back into the grid circuit in such a mannor as to cancel and override the input potential from the circuit DC. Because of this cancellation eflect,'technically known as degeneration, the voltage in the output circuit ceases to respond to increases in input potential amplitude and consequently all amplitude variations above the threshold value, determined by the polarizing potentials applied tothe' diodes, are eliminated. Assume, for example, that the amplification of the stage is 100, then for .1 peak volt radio frequency applied to the grid 4, there will be a voltage across Z of peak volts radio frequency. If the polarizing potentials applied to the diodes is likewise 10 volts, then there will be no degeneration until the voltage across Z exceeds this .value. Any attempt to raise the input voltage above .1 peak volt will not increase the output voltage, because any rise therein over the threshold value immediately results in the feeding back of current through the diodes to the grid 4 in 'phase'opposition to the voltage of the input cir- 'cuit. Thus the diodes operate as a cyclical voltage gate to pass current during such part of any I cycle as the voltage exceeds a predetermined value. v

The purpose of the blocking condensers Ca and Ca isvto keep the D. C. Potential of the plate supply from the diodes and the purpose ofthe radio frequency chokes l8 and 24 is to keep the resistance of the D. C. return path of the diodes at a minimum in order that the system may respond with alacrity. By tying the anode l2 of one diode and the cathode 14 of the other diode together and byapplying negative potential with respect to ground to the anode l8 and positive potential with respect to ground to the cathode 22, the limiter network is made to'respond to both halves of the radio. frequency pulsations.

The circuit of Fig. 2 differs from that of Fig.1

in that the degenerative potentials, instead of being'applied to the control grid of the amplifler, are applied to a separate grid. In Fig. 2

the amplifier tube 2a is shown as a pentode, and

accuses nected as in Fig. 1, however, a tuned circuit for a the output load impedance would not 'be-prac-' ticable because the circuit would have a tendency-- to oscillate. This is because at frequencies below its natural period the tuned circuit will ex-. hibit magnetic reaotance and shift the phase of the-ieed-back voltages applied to the grid so as to make them no longer degenerative.

In Fig. 4 the non-linear impedance is shown as comprising a pair of triodes 32 and 34, and the feed backvoltages are impressed upon the. tank circuit by means of the inductive 'coupling in-' cluding the inductance L5 in the line He. Line I50 is connected at one end to high positive 130- tential indicated on the drawings by "B+ and at the other end to the anodes 36 and 38 of-thej tubes 32 and 34, respectively.- Cathode 40 of tube 32 is grounded while the-grid 42'off'this tube, 1

which is connected through condenser C: to the plate 8, is given a negative bias by connection through a radio frequency choke 44 to the 'ne'gative terminal of a battery 46, the positive tenni nal ofwhich is grounded. The grid 48;. of" tube 34 v is also connected to the negative terminal of' the 5 battery 48. The cathode so oftube 3,4is'connect-fl ed through blocking condenser cawith-theplate 8 and to the ground through the radio frequencyfif choke 52. In Fig. 4 the-output load impedance is. l1 shown as in Fig. 3,"as compnsingthetunedcir- 5 cuit Li, C4.

Inoperation the embodiment of the invention illustratedin Figs. 1 to 3 inclusive, r In Fig. 4, as above described, the, non-linear impedance comprises the separatetriodes 32 andv 34. Obviously the elements of these tubes could 1 be combined in a single tube, as for example, a

- In Figs-1 to'4 the polarizing supplies for the diodes are shown as batteries, but the power supply of the usual radio receiver could as well be used, provided care is taken to keep the internal resistancethereof reasonably low, say of'the order the lead l5a connected to the anode I2 and cathode M of the diodes is connected to a grid 28 of the tube 2a. A radio frequency choke 30 grounded at one end and connected to the lead 15a at its other end provides the D. C. path to ground from the diodes. In Fig. 1 the inductance of the tank circuit LC provided the D. C. path to ground.

- The operation of the circuit of Fig. 2 is similar to that of Fig. 1 and hence requires no additional description.

The circuit of Fig. 3 differs from those of Figs.

' 1 and 2 in that the feed back through the diodes inductively applies the degenerative potentials to the input circuit of the amplifier tubev 2. For this I purpose the lead i5b connected to anode l2 and cathode [4 includes a coil L2 electromagnetically coupled with the tank circuit inductance L. In

the particular circuit of Fig. 3 the output load 4 impedance is shown as a tuned circuit including an inductance L4 bridged by a'condenser C4. The load impedance of Fig. 3 could, if desired, be a resistance such as Z of Figs. 1 and 2, or could be a radio frequency choke. With the feed-back conof a few hundreds of ohms or less.

From the above description of the circuit diagramsof Figs. -1 to 4 it will be apparent that the invention provides an efiicient and rapidly oper-1. ating limiter circuit, for receivers of frequency modulated, radiowaves. Ineach case degeneration of the amplifier input is insured whenever;

the output voltage exceeds the threshold value and .this'degeneration is controlled by a non-- linear impedance in the feed back circuit oper j ating as acyclical voltagegate which opens at output voltages above the threshold-valueto pass current for impressing degenerative potentials on the input circuit. The particular non-linear impedances illustrated in the drawings are-diodes and trio'des but obviously other devi ces,-,such} as, be. substi crystal detectorsor rectiflers, could tuted therefor.

Iclaim:

1. A" limiter circuit for frequency modulated radio receivers comprising in combination an amplifier, input'and output circuits therefor, [said output circuit comprising a resistive impedance', .a feed back circuit interconnecting said input a and output circuits, a one-way conducting element in said feed back circuit, means for. applyor the circuit of 4, currentwill be passed bythe tubes 32 and-34-wh'en the f voltage across the output circuit'isfsumcient' to overcome the negative grid. bias otthe'tubes af- 7 fordedby battery 46.-- 'I'his'curr'ent; throughthe inductive coupling L5, L applies degenerative po-- tentials to the input-circuit of the tube 2 as in through to the input circuit.

inga biasing potential to said element for preventing passage of current therethrough until through until the output voltage in the other direction exceeds said last mentioned biasing potential whereby when the output potential exceeds a predeterminedvalue degenerative potentials are impressed upon said input circuit during both peaks or the voltage wave.

3. A limiter circuit for radio receivers comprising an amplifier, an input circuit and an output circuit therefor, a pair of diodes, means connecting the anode of one diode and the cathode of the other diode together and to said input circuit, means for applying a positive biasing' potential to the cathode of the first mentioned diode and for'applying a negative biasing potential to the anode oi! the last mentioned diode, and circuit connections,- exclusive of said biasing means, between the output circuit and said last mentioned. anode and cathode whereby, when the output voltage exceeds the potentials applied to said diodes, current will he lied back there- 4. The limiter circuit according to claim 3 wherein the degenerative potentials created by the currents passedby the diodesare impressed directly on the amplifier control grid.

5. The limiter circuit according: to claim 3 wherein said amplifier includes a plurality oi gridsand i wherein the degenerative potentials I created by the currentspassedby the'diodes are impressed on a grid of the amplifier other than the control grid.

8. In a frequency-modulation amplifier and limiter circuit a high-gain vacuum tube having input and output electrodes, an input circuit connected to the input electrodes and an aperiodic output circuit connected to the output electrodes; a negative feed-back path connected between said output and input circuits, said path provided by a bilateral independent voltage-gate biased to become substantially 100% conducting when the output voltage exceeds a predetermined level during any half cycle.

9. A limiter circuit for frequency modulated radio receivers comprising in combination a high gain radio frequency amplifierhavlng a cathode, a'control grid and an anode, an input circuit connected across the grid and cathode of said amplifier, an output circuit including a resistive impedance connected to the anode of said amplifier,

a feed back circuit connecting the grid. and theanode of said amplifier, said feed back circuit including two oppositely connected one-way conducting elements, means ior applying a biasin potential .to one of said elements for preventing passage of current therethrough until the voltage across said impedance in one direction ex- 7 ceeds said biasing potential and means for applying a biasing potential to the other of said elements for preventing passage of current therethrough until the voltage across said impedance in the other direction exceeds said last mentloned biasing potential.

10. The limiter circuit according to claim 9, wherein each of said one-way conducting ele-p ments is a diode having one electrode connected directly to the grid of said amplifier and the other electrode connected to the means for apc. The limitercircuit according to claim a a wherein the degenerative potentials created by the currents passed by'said diodes are inductively I impressed uponsaid input circuit.

.77. A limiter circuit to! radio receivers comprising an amplifier, an input circuit and an output circuit'therefor, a feed back circuit beplying biasing potential to that element and to theanode of said amplifier, condensers interposed between'the anode of said amplifier and said last mentioned electrodes oisaid diodes and radio frequency chokes interposed between the biasing, means and the last mentioned electrodes of said diodes.

11. A-limiter circuit 1'01 frequency modulated radio receivers comprising inc'ombination a highgain radio frequency amplifier, inputand output circuits connected therewith, teed-back potential for the anodes of said triodes, the grid or one or said triodes being connected to the output circuit, means for app y a biasing negative potential to the grids of both triodes, the cathode of the other triode being connected to the output circuit and-the cathode of the first V I mentioned triode being grounded, whereby when means connecting said circuits for impressing degenerative potentials on the input circuit when the output voltage during anyv part oifa cycle exceeds a predetermined value, said feed-back means including independent non-linear impedances, means inclllding a source of potential for biasing said impedances to render them nonconducting until the output volt ge exceeds the predetermined value and-means connected with said source for preventing the passage or radio irequency current thereto when said impedances are conducting while providing minimum resistance to the Passa e of direct current.

CARL B. ATKINS. 

